1. Field of the Invention
The present invention relates to a light modulation device, and more particularly, to a light modulation device using a thin film made of material with an electro-optical effect.
2. Description of the Background Art
Conventionally, a waveguide type light modulation device where optical waveguides or modulation electrodes are formed on a substrate having an electro-optical effect, has been much used in an optical communication field or an optical measurement field.
Specially, an information transfer amount has a tendency to be increased accompanied by progress of multimedia and thus a widening of a light modulation frequency is demanded. As means for realizing the widening, an external modulation method by an LN modulator or the like employing LiNbO3 (hereinafter, referred to as “LN”) has been proposed. However, in order to realize the widening of the LN modulator, a velocity matching between a microwave which is a modulation signal and a light wave and reduction of driving voltages are required.
As means for solving such problems, it has been known conventionally that making a thickness of a substrate thin is intended to satisfy a velocity matching condition of a microwave and a light wave as well as to reduce driving voltages.
In the following Patent Document 1 or 2, an optical waveguide and modulation electrodes are formed on a substrate (hereinafter, referred to as “first substrate”) with a thickness of below 30 μm and another substrate (hereinafter, referred to as “second substrate”) with a lower permittivity than the first substrate is bonded thereto, to reduce an effective refractive for the microwave for intending to match a velocity between the microwave and the light wave and further to maintain strength of the substrates.
Patent Document 1: Japanese Patent Application Laid-Open No. S64-18121
Patent Document 2: Japanese Patent Application Laid-Open No. 2003-215519
In the Patent Document 1 or 2, LiNbO3 is mainly used for the first substrate, and material with a lower permittivity than LN, such as quartz, glass, alumina or the like is used for the second substrate. A temperature drift or a DC drift accompanied by a temperature variation occurs in a combination of such material by a difference of a coefficient of linear expansion therebetween. Patent Document 2 discloses that, in order to remove such inconvenience, the bonding of the first substrate and the second substrate is performed by use of an adhesive with a coefficient of linear expansion close to the first substrate.
However, comparing a modulator using the LN substrate which has been manufactured conventionally with a modulator to make the LN substrate thin, as the substrate becomes thinner, light emitted or leaked from the optical waveguide, or light and the like incident on other than the optical waveguide from an optical fiber for incidence (hereinafter, such light is referred to as “faint light.”) has a strong tendency to be confined within the substrates. In the conventional LN substrate, since the substrate is thick (e.g., 500 μm to 1000 μm), there are sufficient areas which do not influence the waveguide (e.g., several μm in depth) and a spatial distribution density of light filled with the faint light (hereinafter, referred to as “faint light density”) becomes lower. As a result, the faint light has little impact. However, when a thickness of the substrate is the same as a distance in a depth direction of the waveguide, since a faint light density propagating in a direction parallel to a surface of the substrate within the substrate becomes higher, the faint light propagates within the substrate to be re-incident on the optical waveguide or to be incident on an optical fiber for output connected to a light modulation device, resulting in deteriorating a S/N ratio of output light.
In addition, when a thin substrate is used for a light modulation device, since the thin substrate cannot give a sufficient strength, the thin substrate which is the first substrate and the second substrate are required to be bonded by a supplementing plate by use of an adhesive. In this case, when a refractive of the adhesive is lower than that of the thin substrate, the confinement of the faint light is more outstanding. For this reason, this applicant has proposed in Patent Document 3 that a light absorbing portion, a high refractive portion, a light guide portion or a concave portion is formed in an area excluding the optical waveguide and a periphery thereof in the thin substrate, to remove the faint light within the thin substrate.
Patent Document 3: Japanese Patent Application No. 2005-96447 (Application Date: Mar. 29, 2005)
However, like Patent Document 2, there is a problem in that, when a surface, which is opposite to the thin substrate, of a adhesive layer of the supplanting plate is smoothed, the faint light emitted to other than the optical waveguide is reflected on the surface of the supplementing plate to be re-incident on the thin substrate, thereby recombining with the optical waveguide.